Many applications require high-power laser sources with short pulse duration, narrow spectral bandwidth, low intensity/phase noise, compact size, and low cost. A common technique to achieve narrow, e.g., Fourier transform limited, bandwidth of high power and low noise radiation is injection seeding. A basic requirement for effective injection seeding is that resonance between the slave modes and the photons from the master must be kept in every pulse. Conventionally, the master-slave resonance is based on stabilized wavelength of the seed laser (master), active control of the resonance wavelength or longitudinal modes of the seeded laser (slave), and locked phase angle between the injected and output signals.
One way to stabilize seed laser wavelength is by use of filtered optical feedback. As an example, in U.S. Pat. No. 5,809,048, Shichijyo et al. used an external wavelength sensitive optical device and a birefringent Lyot filter for improving the wavelength stability. Another way to accomplish the wavelength stabilization of a semiconductor laser was disclosed in U.S. Pat. No. 4,583,228, wherein the drive current and the laser temperature were controlled by feedback signals derived from an external Fabry-Perot interferometer. Alternatively, the wavelength reference can be located within the oscillator, as described in U.S. Pat. No. 6,930,822. Wavelength stabilization can also be accomplished by movement of an optical element, e.g., rotation of a prism inside the laser, together with a signal processor. An example of such systems is given in U.S. Pat. No. 6,393,037. Other means of wavelength stabilization includes adjusting the temperature or angular tilt or spacing of an intracavity etalon; or adjusting the angle of a prism, a grating, a mirror, or a birefringent filter; or adjustment of the cavity length.
Active control of the resonance wavelength or longitudinal modes of a seeded laser oscillator to match the injected wavelength within the necessary tolerance typically requires modifications of the oscillator cavity by mounting a piezoelectric transducer (PZT) or a magneto-electric device onto a mirror, which dithers according to the feedback signal generated from a control system. A practical implementation of such systems can be found in, e.g., Applied Optics 35 pp. 1999-2004 (1996).
In a Q-switched laser operation, triggering can be controlled to occur only when the interference of the seed light and the light that leaks out from a slave cavity mirror shows a maximum. This technique guarantees that Q-switch is triggered only when the slave cavity is in resonance with the seed laser. Pioneered by Fry and co-workers, this technique has a disadvantage, namely, the laser could fire at any time during the voltage ramp, consequently, synchronization with other events might be impossible.
This problem can be overcome by triggering the Q-switch at a predefined time after the start of the ramp. Before reaching this predefined time and once the resonance is detected, the ramp is stopped and the length of the slave cavity is held constant. This method guarantees that laser shot occurs at a fixed time. However, due to the need to hold the ramp, ramping times have to be reduced in order to avoid mechanical ringing in the system. An application of the ramp-hold-fire seeding technique to a Ti:sapphire laser is described in Applied Optics 40, pp. 3046-3050 (2001).
An alternative method for master-slave resonance is based on minimizing the build-up time of the laser radiation. Many commercial Nd:YAG systems use this technique. An obvious problem of this technique is that the direction of deviation from the optimum cavity length is not measurable and the feedback occurs in a random fashion. In practice, this technique only works reliably for a predefined and carefully optimized repetition rate, between 10 Hz and 100 Hz. Refer to, e.g., Applied Optics 25 pp. 629-633 (1986).
Conventional injection-seeding systems are inevitably complicated, expensive, and inconvenient in use. In addition, PZT- or magneto-electric device-mounted cavity mirrors have to be separated from the gain media, making monolithic structure of slave oscillators impossible.